1. Field of the Invention
The invention relates to an optical pickup apparatus for reading information from a plurality of different kinds of recording media by emitting a plurality of laser beams having different wavelengths and relates to a laser diode chip for the optical pickup apparatus.
2. Description of the Related Art
Generally, a semiconductor laser device is used as a light source of an optical pickup apparatus for playing an optical information recording medium such as CD, DVD, or the like.
To play back the recording medium, the light emission wavelength and the numerical aperture (NA) of an objective lens of the semiconductor laser device which is used for playing a CD and for playing a DVD are different from each other. For example, in the case of the DVD, the wavelength is equal to 650 nm and the NA is equal to 0.6 and, in the case of the CD, the wavelength is equal to 780 nm and the NA is equal to 0.45.
To play different kinds of discs such as CD, DVD, and the like by one disc player, therefore, an optical pickup apparatus having therein light sources of two wavelengths of 650 nm and 780 nm is being used. FIG. 1 shows an example of the optical pickup apparatus.
According to the optical pickup apparatus shown in FIG. 1, a laser device 1 for emitting a laser beam having a wavelength of 650 nm, a laser device 2 for emitting a laser beam having a wavelength of 780 nm, a synthesizing prism 3, a half mirror 4, a collimator lens 5, and an objective lens 6 are sequentially arranged. Further, a photodetector 7 is arranged on another optical axis which is branched from the half mirror 4. In the construction, since an optical system starting with the synthesizing filter 3 and extending to an optical disc 8 is used in common for the CD and DVD, in both cases, the light emitted from the laser device passes through the synthesizing filter 3 and, thereafter, is guided toward the optical disc 8 along an optical axis Y. The objective lens 6 used here is a lens having double focal points and different focal positions can be obtained in accordance with two wavelengths. A spherical aberration which is caused by different thicknesses of surface substrates of the CD and DVD can be, consequently, suppressed.
In the construction, however, since a synthesizing prism or the like is needed, a large number of parts is required and the costs of production are high. Further, because it is necessary to match the positions of the two laser devices and the synthesizing prism, the construction becomes complicated, and it is difficult to make adjustments to the device.